#include <cmath>
#include <iostream>
using namespace std;
double point[1000000][2],A;
int m;
double det(double x1,double y1,double x2,double y2)
{
    return x1*y2-x2*y1;
}

double area()
{
    int i;
    float temp=0;
    for(i=0;i<m-1;i++)
    {
        temp+=det(point[i][0],point[i][1],point[i+1][0],point[i+1][1]);
    }
    temp+=det(point[i][0],point[i][1],point[0][0],point[0][1]);
    return temp/2;
} 
int main()
{
    int i;
    while (scanf("%d",&m)!=EOF)
    {
        if (m==0)break;
        for (i=0;i<m;i++)
        {
            cin>>point[i][0]>>point[i][1];
        }
        printf("%.1f\n",area());
    }
    return 0;
}
